1. Field of the Invention
The present invention relates to magnetic bubble domain devices, and in particular mask patterns or configurations of guide structures for the propagation of magnetic bubble domains.
2. Description of the Prior Art
There are various known devices and propagation structures which provide means for propagating magnetic bubble domains on a layer of material. One of the most important of such arrangements for propagation of bubbles is the so called "field access" configuration which utilizes specifically shaped elements of a magnetic material (typically permalloy), which when subjected to a rotation or reorienting drive magnetic field parallel to the plane of the layer of magnetic material supporting the bubbles, produces a propagating series of potential wells in the layer which causes the bubble or bubbles present therein to propagate synchronously with the potential wells. Additional bias field is also typically provided normal to the magnetic layer of material to stabilize the bubbles. The rotating magnetic field usually consists of a magnetic field rotating about an axis parallel to the bias field.
The fabrication of circuits for the propagation of magnetic bubble domains has used most of the same fabrication techniques as is employed in integrated circuits, such as the selective deposition and/or etching of material through masks of desired patterns. One of the basic limitations of the density of the magnetic bubble devices is the size of the field propagating elements or patterns themselves, which is dependent upon the lithographic resolution of the fabricating process.
Over the last few years, bubble propagation patterns have evolved from the basic T-I bar structure that requires a resolution of about 1:16, to the present "state-of-the-art" gap tolerant patterns such as the asymmetric chevron in which the resolution is about 1:8. The resolution is defined as the ratio between the minimum feature size to the circuit period length. With the photolithographic resolution at about 1 .mu.m minimum feature a bit density of about 10.sup.6 bits/cm.sup.2 is presently achieved, using the gap tolerant pattern.
To achieve lower device cost through higher bit density it is necessary to employ propagation patterns having a lower resolution. Some of the approaches that are being developed are the ion-implanted propagation pattern (I.sup.2 P.sup.2), also known as contiguous disk, and the lattice file. The I.sup.2 P.sup.2 approach promises a resolution of less than 1:4, thus quadrupling the present bit density. However, the I.sup.2 P.sup.2 devices presently have a number of disadvantages. The development of some of the essential device functions, such as the detector in I.sup.2 P.sup.2 devices has not been satisfactorily concluded. The bubble lattice file has the potentiality of 10.sup.7 bits/cm.sup.2, but requires very complex processing, therefore making such devices prohibitively expensive.
Another configuration also using a contiguous disk arrangement is shown in U.S. Pat. No. 4,151,606, although such an arrangement is applied to the propagation of inverted Neel wall sections instead of magnetic bubbles. The disadvantage of such technology is that the application of bubble devices to digital data storage requires redundancy to ensure reliability, which is not easily implemented.
Prior to the present invention there has not been a high density, permalloy magnetic bubble domain guide structure that is easily fabricated.